1. Field of the Invention
The teachings herein relate to reducing eccentricity and out-of-roundness in a turbine, and, in particular, to techniques for controlling cooling.
2. Description of the Related Art
Air impingement cooling has been used to manage the casing temperature of small gas turbines and to reduce and maintain the clearances between rotating blades and accompanying interior casing surfaces. One problem for air impingement cooling systems on gas turbines is the ability to achieve a uniform heat transfer coefficient across large non-uniform non-standard casing surfaces. On small gas turbines, small impingement holes and short nozzle to surface distances are normally applied. These factors produce the required higher heat transfer coefficients on the casing. One detrimental impact of applying small impingement cooling holes is the need for operating with high differential pressure drop across the holes. This results in the requirement for undesirable high cooling air supply pressures which negatively impacts net efficiency.
Impingement cooling has been applied to aircraft engines as a method of turbine clearance control. However, the impingement systems used on aircraft engines cannot be used in some turbine applications. The systems applied to aircraft engines use air extracted from the compressor as the cooling medium. It is not feasible to use compressor extraction air on some other gas turbines because the design heat transfer coefficients require cooler air temperatures. For example, some other gas turbines have a significantly larger, non-uniform casing surface that requires an intricate manifold design as compared to aircraft engines. Also, the casing thickness and casing thickness variations are considerably greater on such gas turbines.
A problem with eccentricity and out-of-roundness for the gas turbine casing is rub between the rotor and the stator and other mechanical problems. This decreases efficiency of the power output. The problem is primarily addressed by increasing the cold built clearance of the gas turbines, which does not address the problem, but further lowers the efficiency.
Some apparatus are known that provide cooling with impingement air. For example, one system includes an impingement cooling manifold affixed to a casing of the turbine, wherein the impingement cooling manifold includes a plurality of impingement holes in the surface of the impingement cooling manifold; and a blower that provides air flow across the plurality of impingement holes of the impingement cooling manifold to cool the casing of the turbine to control the clearance between a tip of a turbine blade and a shroud of the turbine. While such a system has certain advantages, it is desirable to provide greater control over cooling to further improve performance.
Thus, there is a need for an impingement cooling system that can provide clearance control on gas turbines. Preferably, this system delivers the required heat transfer coefficient to the targeted casing surface, and provides a high degree of control over the flow of air to the casing. Such a system is disclosed herein.